METAMERISM. 25 1 



as the latter rami do not bear so constant a relation to the 

 inter-muscular space laterally of the dorsal musculature 

 as the dorsal rami to the space mesially of it, their anas- 

 tomoses with the r. lateralis are neither so extensive nor 

 so constant as the others. 



II. — Metamerism. 



Certain questions of metamerism and the relations of 

 the components to each other remain to be considered. 



Our conception of the composition of the primitive 

 segmental nerve will depend somewhat upon whether we 

 regard the cranial (branchiomeric) or the spinal type as 

 the more primitive. But in either case we may assume 

 with great probability that the typical segmental nerve in 

 the earliest vertebrate contained somatic sensory, or 

 general cutaneous fibres, also somatic motor for the 

 voluntary musculature and viscero-motor and viscero- 

 sensory components. In the head the form which the 

 segmental nerve takes is dominated by the branchio- 

 merism, and we are justified now in concluding that the 

 original branchiomerism coincided with the metamerism 

 (see especially Neal, '98). 



We have suggested in the preceding sections the follow- 

 ing comparisons between the components of the head and 

 those of the trunk. The general cutaneous and somatic 

 motor of the cranial nerves are strictly homodynamous 

 with the corresponding components of the spinals. The 

 viscero-motor of the trunk is present in the same form in 

 the head, but has also suffered an extreme differentiation 

 and very profound modification in connection with the 

 branchio-motor apparatus. The rudimentary visceral 

 sensory system of the trunk is represented by the com- 

 munis system of the head and has also suffered great 



